Great interest exist in commercializing solar cells to replace electrical power presently generated by coal or gas power generation plants. Coal contributes to the generation of carbon dioxide and other pollutants in the atmosphere. Natural gas is considered less polluting but still generates carbon dioxide and is also becoming very expensive.
A solar cell is a photo diode including a semiconducting p-n junction, perhaps sandwiching an intrinsic layer to provide most of the photon conversion, and having electrodes on either side to extract the electrical power derived from the photo-electric conversion within the solar cell. Solar cells were largely developed following the lead of electronic integrated circuits in which a large-size planar photo diode is formed in a crystalline substrate of silicon or less frequently gallium arsenide. Gallium arsenide has always been expensive and even thick plates of crystalline silicon have become increasingly expensive. Accordingly, the technology has been explored of thin film solar (TFS) cells in which semiconducting layers are deposited on a large foreign substrate, which may be glass, steel, or even plastic. The deposition technology is akin to that used for flat panel displays, such as LCD and plasma television screens formed on glass substrates.
A fundamental problem with TFS cells is that at least one of the electrodes must be relatively electrically conductive but optically transparent. Mesh metallic electrodes have been used for crystalline solar cells but their efficiency is degraded by geometrical factors and the mesh must be photolithographically defined. Electrodes of transparent conductive oxides (TCO) have become popular for terrestrial solar cells. The most prevalent TCO is indium tin oxide (ITO) widely used for flat panel displays, but indium is relatively expensive and not conducive to widespread or large-scale power generation. Tin oxide and zinc oxide have also been considered as TCO materials.
Zinc oxide has been the favored substitute TCO material. However, to date its performance has not been optimal since it needs to satisfy a number requirements. It needs to be highly conductive as well as highly transmissive. It needs to provide a good interface to the adjacent semiconductor material. Its refractive index should be tailored to reduce any reflectance of incident light due to a steep change in the refractive index at its two interfaces. It is desirable that the TCO be deposited by sputtering. Sputtering from a metal oxide target can be achieved either with RF sputtering or DC sputtering. Generally, RF sputtering is disfavored over DC sputtering because of its expense and low sputtering rate. However, the relatively low conductance of a metal oxide target seems to force the use of RF sputtering. Suggestions of reactive sputtering from a metal target in an oxygen ambient have not been supported by experimental verification of good product.